1. Field of the Invention
The present invention relates to a molding method for an optical element, where a glass material is heated to be softened and then press-molded within a mold.
2. Related Background Art
Recently, there has been developed a method in which a glass material for an optical element, such as a glass blank pre-formed to have some degree of surface accuracy and configuration, is put in a mold having a given surface accuracy, and press-molded while heating to thereby produce an optical element having a high-accuracy optical functional surface, with no need for post-processing such as grinding and polishing.
Generally, in such a press molding method, upper and lower molding members are disposed vertically to be opposed to each other so as to be slidable in a shell mold. A glass material to be molded is introduced into a cavity defined by the upper, lower, and shell molding members, and these molding members are heated to a given temperature suited for molding, i.e., a temperature at which the glass material shows a viscosity of 10.sup.8 to 10.sup.12 dPaS, depending upon materials, and subsequently the mold is closed and the introduced glass material is pressed for an appropriate amount of time, so that the shape on the molding surfaces of the molding members are transferred onto the surface of the glass material. In this case, upon molding the environment therein is made to be a non-oxidizing atmosphere such as a nitrogen atmosphere in order to prevent oxidation of these molding members.
Thereafter, the molding members are cooled to a temperature sufficiently close to the transition temperature of the glass material to be molded, and then the mold is opened and the pressing pressure is released to take out a molded optical element.
Prior to the introduction into the mold, the glass material to be molded may also be pre-heated to the appropriate temperature, or after heating to a temperature suitable for molding, the glass material may also be introduced into the mold. Moreover, continuous molding is performed in such a manner that the glass material to be molded, together with the molding members, are conveyed to a pressing machine, and then are respectively heated at given locations and are press-molded by the pressing machine, or are further cooled. These methods can speed molding.
The above-described optical element press molding methods have been disclosed by U.S. Pat. Nos. 3,833,347 and 3,844,755 and Japanese Patent Application Laid-Open No. 58-84134 and others, wherein a glass material is introduced in advance into a mold and subsequently the molding members and the glass material are heated to an isothermal condition and the press molding is completed at a given temperature. In addition, Japanese Patent Application Laid-Open Nos. 59-203732 and 62-27334 and others disclose a method in which a glass material heated to a temperature suited for molding is transferred into a mold whose temperature is maintained below the glass material temperature.
However, the above-described prior art contain the following problems. First of all, in a method disclosed by Japanese Patent Application Laid-Open No. 58-84134 and other, when a glass material is placed in advance within a mold where molding members and the glass material are heated to an isothermal condition, the proper range of temperature for press-molding corresponds to a glass viscosity of 10.sup.9 to 10.sup.9.5 dPaS.
This is because, when the temperature is below a value corresponding to 10.sup.9.5 dPaS, the glass is broken or a long time is required for the deformation on press molding, thus such temperature is not practical from a productivity standpoint, on the other hand, when the temperature is more than that equivalent to 10.sup.9 dPaS, poor conditions such as fusion to the mold surface and cloudiness on the transferred surfaces is apt to occur. Accordingly, the general pressing time is approximately 30 seconds to 10 minutes, and the cycle time becomes considerably longer because, after the completion of the deformation of the glass material by pressing, the glass material is cooled to within a temperature range in which its configuration does not change, and then released from the mold.
Furthermore, Japanese Patent Application Laid-Open No. 59-203732 discloses a method in which a glass material is placed on a holding member and heated to a temperature corresponding to its viscosity of 10.sup.5.5 to 10.sup.7 dPaS, and is pressed by a molding member whose temperature is kept 100.degree. C. lower than the glass temperature.
However, in the case that the glass material is heated on the holding member to the aforesaid temperature, following problems will arise. The glass deforms into an improper shape when molded, so that a gas remains in a surface pressed, or the holding member intrudes into the glass, so that the optical element cannot attain a necessary accuracy, or when the glass material heated is transferred into the mold, the glass temperature falls to make press molding impossible.
In addition, since the circumferential portion of the glass material held by the holding member contacts with the holding member, its surface becomes rough and difficult to use as an optical functional surface. Further, since the glass is pressed in a state of being placed on the holding member, the positioning becomes difficult and burrs tend to appear on the molded part. Still further, the molded part is adhered to the holding member, and hence it becomes difficult to release it from the mold. Depending upon the shape, such as for a convex lens, the difficulty will increase.
Moreover, Japanese Patent Application Laid-Open No. 62-27334 discloses a method in which a glass material is press-molded while the glass material is heated to a temperature corresponding to a viscosity of 10.sup.6 to 10.sup.8 dPaS and the molding members are set to Tg to (Tg-200.degree. C.). Tg is transition temperature. However, since the temperature of the molding members are too low, the glass temperature rapidly decreases when press-molded, with the result that an accuracy necessary for the molded part is unobtainable, besides the pressing is not achievable to produce a predetermined thickness, the glass is broken, or the difference in temperature distribution within the glass takes place, so that the surface of the molded part wrinkles. In addition, as well as the aforesaid well-known example, the problems will occur in carrying out the press molding in a state that the glass material is placed on the holding member.